There are many applications that need optical scanners to convert objects, such as text and graphics objects on paper-based materials, for example, to an electronic format that can be subsequently analysed, distributed and archived. One of the more popular type of consumer optical scanners is flatbed scanner type device that converts objects, including pictures and papers, to image data that can be used, for example, for building Web pages and optical character recognition. The optical scanners may be referred to as image scanners as the output thereof is generally in digital images.
Similar to a copy machine, a flatbed scanner generally includes a cover or lid and a glass panel. Generally a scanning document must be placed on the glass panel face down with the lid closed, so that an image sensing mechanism or imager in a compartment located beneath the glass panel can properly scan the scanning document. The image sensing mechanism generally includes one or more light sources, such as LEDs arranged below the glass panel that illuminate the scanning document. The light that is reflected from the scanning document is received by an image sensor, such as a charge coupled device (CCD), a contact image sensor (CIS) or a CMOS sensor.
Whilst such known flatbed scanners are suitable for many applications, the colour accuracy of data generated by such scanners can be improved. One way to improve colour accuracy is to increase the number of colour channels that are captured by a scanner. Generally speaking, a captured image contains signal data across a colour spectrum. An individual colour channel may hold some signal related to most or all parts of the colour spectrum, but each channel will hold signal that has a different weight at different parts of the spectrum. One channel may be biased towards greenish colours whereas another may be biased towards reds or blues, etc. To represent a colour we treat each channel as a dimension. Similar shades of red, for example, would be represented as a volume defined by three number ranges, one range in each of the three channels. A set of colour channels that represents a particular gamut of colours is generally referred to as a colour space. Generally two or three colour channels are used to represent a colour space, although other colour spaces having other numbers of colour channels are known. One way of increasing the number of captured colour channels to improve the colour accuracy of flatbed scanners has been disclosed in our co-pending U.S. patent application Ser. No. 11/263,482 entitled “Image Optimisation System and Method” filed the 31 Oct. 2005, the disclosure of which is incorporated herein by reference. U.S. patent application Ser. No. 11/263,482 uses two separate light sources, or groups of light sources, with different spectral properties. Thus a first set of colour channels is recorded by the image sensor when the first light source is used and a further second set of colour channels is recorded by the image sensor when the second light source having the different spectral properties is used. The two scans can be used to provide the additional spectral information required to give improved colour accuracy.
However, whenever multiple images having different captured colour channels are combined there is a risk that the illumination levels between the images are unequal, either through the use of different light sources (as in the above-mentioned US patent application) or changing illumination levels between each image being captured. The inconsistencies between the captured images may give rise to undesirable anomalies in the combined image. The present invention seeks to ameliorate these anomalies.